Such phosphors are disclosed in U.S. Pat. Nos. 4,225,653 and 4,387,141. In both patents the phosphor composition has the formula YNb.sub.x Ta.sub.1-x O.sub.4, where x is 0 to about 0.15.
It has been discovered that phosphor brightness can be improved by the addition of small molar amounts of a halide of rubidium, cesium, strontium, or barium, and, optionally, an oxide of gallium or aluminum.